Television in Color (American Cinematographer Magazine June 1931)

 

Television in Color from Motion Picture Film

by HERBERT E. IVES
Bell Telephone Laboratories, New York.

From an American Cinematographer Magazine June 1931

IN SPECULATIONS on the possible uses for television, one project which receives considerable attention, partly because of its relative ease of accomplishment, is the transmission of images from motion picture film. It is true that the practical simultaneity of event and viewing, which is the unique offering of television, is lost when the time necessary for photographic development of the film intervenes. Nevertheless it is conceivable that if this delay is small, television from film may still possess such an advantage over the material transportation of film as to give it a real field.
A further possibility, more remote, but within the range of legitimate speculation, is that
television apparatus may sometime be used to receive, in the home, motion pictures of the sort now offered in the theatres or in home projection outfits. However distant these mergings of the two arts may be, the technical problems presented are pretty clearly defined, and offer interesting features for study.

Among these problems is the transmission of images in color from colored motion picture film. This paper describes a method of accomplishing this, using the receiving apparatus for television in color recently described, and special sending apparatus which utilizes the latest form of colored moving
pictures—the ridged film now marketed under the name of Kodacolor.
As an introduction to the method of telecinematography in color using ridged film, it is profitable to outline how the problem could be solved with film in which the colors are incorporated by dyes (e. g., Technicolor), and the three-color transmitting and receiving system recently developed in the Bell
Laboratories. *1
This may be done most easily by considering Fig. 1, where the three-color transmitting apparatus is shown in section, with the addition of film handling means.

The photoelectric cell cabinet, containing three sets of color-sensitive cells with appropriate filters, is indicated at C, from which three communication channels, R, G, and B, carry the red, green, and blue signals to the receiving end. At A is the arc lamp, whose light is condensed upon the perforated disk D,
which is driven by the synchronous motor M. The lens L projects an image of the disk upon the matte white screen S, from which light is reflected back into the photoelectric cells. The film F, as it unwinds from the reel R1 onto the reel R2 passes in front of the disk D, and as closely as is practicable to it so
that the film and the disk holes are in focus together on S.
If, with the apparatus as just described, the film stands still, with a picture frame exactly filling the field aperture in front of the disk, and the disk rotates at its normal speed for television, the screen S shows a projected image of the film, colored if the film is colored, and capable of being picked up by the photoelectric cells and transmitted, to be received like the image of a colored object by the single disk, three-lamp receiving apparatus, as ordinarily used for this purpose. When the film is moved in order to give a motion picture, there are two alternative forms of scanning disk available, depending on whether the motion of the film is intermittent, as in most cameras and projectors, or continuous. In the first case, a scanning disk must be used with a blank sector corresponding to the period occupied by the shift of the film between frames, as shown in D1, Fig. 2, and a similar disk must be used at the receiving end also.

The use of intermittent exposures is, however, not only inefficient, because of the waste of line-time during the blank period, but is quite unnecessary when the image is analyzed by successive passages of a scanning aperture across the field. Instead of a disk provided with a spiral of holes it is simpler and better to use a disk with the scanning holes arranged in a circle, as shown at D2, Fig. 2, and to give the film a uniform and continuous motion along the vertical diameter of the disk. When this is done the screen S shows merely a horizontal strip of light (indicated in Fig. 1 by the solid line) but the usual spirally-perforated disk at the receiving end spreads this out into a complete picture.
This method of transmitting colored images from motion picture film, while completely practical, suffers under the disadvantage that it requires an original colored film of a sort which is both expensive and time-consuming to produce. Should television transmission from film become popular it is probable
that the chief demand would be for films which would be shown but once, and for showings within a few hours, at most, of the event. Some form of colored film would then be called for which could be prepared quickly and cheaply, and the film process need not be one adapted for making numerous copies.
A form of colored motion picture which very completely meets these requirements is produced by the Kodacolor process.*2

In this the image is black and white, but is distributed into a triple linear mosaic by lenticular ridges on the film. Exposure is made through a lens with three apertures, and projection is accomplished through a lens similarly equipped with three apertures, covered with red, green, and blue filters. The original negative, made into a positive by a process of photographic reversal, is used for projection. There is but one film available, but this is all that is necessary for the use in television which we are considering. The film is cheap as compared to a film in which the color is added by a dyeing process,
and the time required to prepare it for projection is a matter of hours instead of days.

The method of using Kodacolor film may be most comprehensively described by saying that the film is to be projected as though for display upon a screen, but that the three beams of light issuing from the projection lens are directed each into a separate photoelectric cell for television transmission. With
the details of the apparatus shown in Fig. 1 in mind, the Kodacolor film arrangement is readily grasped from Fig. 3, where the upper view (a) shows the elevation, the middle view (b) the plan, and (c) shows a detail of the scanning disk and film.

Starting with the light source A, the light is condensed by the condenser system C on the film F which moves continuously past the slot S and directly behind the disk D. The disk is shown as provided with radial slots R, these together with the fixed slot S forming the scanning holes. After passing through the film and disk the light is projected as if to a screen by the lens L, in front of which is placed, in the regular projector, the set of red, green, and blue filters T. For our purpose both the screen and the filters are dispensed with. After passing through the lens, the light is diverted into three photoelectric cells,
P1, P2, and P3, by the mirrors M. These cells are all similar, and need not be color-sensitive. The filters are omitted as obviously unnecessary—color is not needed until the signals are received and recombined at the receiving disk where the same apparatus is used as for the reception of signals from
original colored projects.

The arrangement of apparatus shown in Fig. 3 calls for the slot, film, and disk being practically in contact. This condition, which must be met if color fringes are to be avoided, is likely to offer some difficulty, since both are moving at high speed. An alternative arrangement, by which the disk and film are separated, is shown in Fig. 4. Here the symbols are as in Fig. 3, and the apparatus is the same from the lamp A to the film F. The disk is, however, removed to a new position beyond the projection lens L1, which is supplemented by a shortfocus lens so that an image of the film F, where it lies over slot S, is projected onto the disk. A third lens L3, close to the disk, images the three apertures T onto mirrors M and photoelectric cells P as before. By this means the film image may be placed accurately in the plane of the disk and color fringes avoided.*3

Additional advantages are that the disk may be made of any convenient size, and that the radial slots to which one is practically driven by constructional difficulties in the very small disk may be replaced by holes as shown at (c).
In describing the apparatus for achieving television in colors by a beam-scanning method’ emphasis was placed on the fact that the same single scanning disk was used at each end as for monochrome work. A similar characteristic holds for the film apparatus here described. Either color or monochrome film can
be used interchangeably, the latter requiring but one transmission channel. If monochrome receiving apparatus only is available when multichrome film is used, it may be received as monochrome, preferably selecting the green channel as giving nearly orthochromatic effects. If three-color receiving apparatus is available of the form previously described *1 images from
monochrome film may be received on all three (red, green, and blue) lamps together, adjusting their relative intensities to give white or any other desired color for the resulting monochrome image.

References
*1 Ives, H. E.: “Television in Color by a Beam-Scanning Method,”
lour. Opt. Soc. of America, 20 (January, 1930), No. I, p. 11.
*2 Photographic Journal (September, 1929), p. 402.
*3 The disk and film could be similarly separated in the form of
apparatus shown in Fig. 1 although the necessity is not so apparent.
(The above article appears here through the courtesy of The Journal
of the Society of Motion Picture Engineers—Editor.)